There are a number of fundamental limitations with well-known conventional conveyor systems which employ a belt for transporting pallets between processing stations. First, the speed of the belt is typically quite limited. This is largely due to the fact that the pallets are typically stopped, e.g., in order to be processed at a processing station, by mechanical stop mechanisms. Thus, if the belt conveyor is operated at a high speed, the strong impact between a pallet and mechanical stop is likely to jar whatever parts the pallet may be carrying for processing. Second, it is generally not possible to vary the acceleration and velocity profiles for individual pallets. For instance, if a first pallet is empty and a second pallet is loaded with delicate parts, it is generally not possible to aggressively accelerate the first pallet to a high speed while controlling the second pallet using more gentle acceleration and velocity profiles. This limitation affects the latency and possibly the throughput of the manufacturing line. Third, the belt conveyor is typically not bidirectional, which may result in a suboptimal design of the manufacturing line. Fourth, the belt conveyor typically provides limited flexibility or programmability, such as being able to very quickly change the positions of processing stations. Finally, the data acquisition capabilities provided by the belt conveyor are typically quite limited. For example, it is typically not possible to know where the pallets and their constituent loads are located along the conveyor at all times. Thus, for instance, it may be difficult to know how many pallets are queued at a particular processing station. For these and other reasons, a conveyor system having multiple moving elements or pallets under substantially independent control may be desirable for various types of applications.
Conveyor systems having multiple pallets under substantially independent control are known in the art, but suffer from a variety of limitations. For example, U.S. Pat. No. 4,841,869 issued Jun. 27, 1989 to Takeuchi et al. discloses a conveyor system utilizing a linear induction motor, comprising a conveyor cart and a guide rail for movably supporting the conveyor cart. The guide rail includes primary coils, and the conveyor cart includes a flexible secondary conductor extending longitudinally of the cart so as to follow the guide rail. The primary coils comprise a station primary coil disposed at each loading and unloading station for stopping and starting the conveyor cart, two primary coils adjacent opposite ends of the station primary coil for decelerating the conveyor cart that is to be stopped at the stat ion by the station primary coil and for accelerating the conveyor cart having started from the station to a target running speed, and a plurality of intermediate accelerating primary coils disposed between two adjacent stations for accelerating the conveyor cart to maintain the latter at the target running speed.
A major shortcoming with the Takeuchi et al. system is that the carts or pallets thereof cannot be positioned to stop at any point along the conveyor, but only where the linear motors thereof are disposed. This makes changing the location of a station a troublesome endeavour. In addition, the system is not capable of pinpointing the location of a moving pallet at any time. In view of these limitations, the Takeuchi et al. system does not feature truly independent and total control of multiple moving elements.
U.S. Pat. No. 5,023,495 issued Jun. 11, 1991 to Ohsaka et al. discloses a moving-magnet type linear d.c. brushless motor having plural moving elements disposed for motion along a track. The track includes a coreless stator armature having a plurality of contiguously arranged coils thereon. Each moving element includes a thrust-generating field magnet having P contiguous magnetic poles of alternating N and S polarity (i.e. polypolar magnet) having one side facing the stator armature. Each moving element may also include a polypolar position-detecting magnet. The track includes a row of position/commutation sensors, each row of position/commutation sensors being provided for detecting the magnetic poles of only the position-detecting magnet of a corresponding moving element. The position/commutation sensors are used in control circuitry for generating an electric current in the stator armature to move the moving elements in predetermined directions separately and independently.
The Ohsaka et al. system also has a number of shortcomings, particularly with respect to the modularity or scaling properties of the system. First, due to the fact that a separate track of position/commutation sensors is required for each moving element, the system can only accommodate a relatively small number of moving elements. Second, the length of the linear motor is limited by a servocontrol mechanism, described as a single microcomputer, which can only process and accommodate a limited number of the position/commutation sensors and associated electric current generating control circuitry. Third, use of the magnetic position-detecting elements provides a relatively poor resolution for measuring the position of the moving element. Fourth, the winding arrangement of the stator armature is essentially that of a linear stepper motor, which presents an uneven magnetic reluctance along the stator armature resulting in relatively noticeable cogging effects and a jerky thrust production. Finally, the, coreless design of the stator armature also results in a relatively low average thrust production which may not be suitable for typical conveyor system applications.